1. Field of the Invention
The present invention relates to an exhaust gas purifying device for an engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 63-283727 discloses an exhaust gas purifying device for an engine in which an exhaust gas purifying catalyst is arranged in an exhaust passage of the engine, the catalyst being capable of reducing nitrogen oxide NO.sub.X included in the inflowing exhaust gas in an oxidizing atmosphere, and in which a reducing agent (hydrocarbon) is secondarily fed to the exhaust passage upstream of the catalyst, to thereby purify NO.sub.X in the catalyst using the hydrocarbon.
It is well known that a catalyst having a porous structure, such as a zeolite catalyst, adsorbs and desorbs hydrocarbon. Thus, NO.sub.X could be reduced in the catalyst by both hydrocarbon in the inflowing exhaust gas and that desorbed from the catalyst. However, the inventors of the present application have found that the NO.sub.X reduction by hydrocarbon desorbed from the catalyst is superior than that by hydrocarbon in the inflowing exhaust gas. Accordingly, in the case where the catalyst has the hydrocarbon adsorption and desorption function as mentioned above, NO.sub.X is sufficiently reduced by causing the catalyst to adsorb a large amount of hydrocarbon therein, or by causing the catalyst to desorb a large amount of hydrocarbon therefrom.